1. Field of the Invention
The invention relates to a liquid crystal display device and a method of fabricating the same.
2. Description of the Related Art
A reflection type liquid crystal display device reflects an incident light at a reflection electrode formed therein towards a viewer. Accordingly, a reflection type liquid crystal display device is not necessary to include a light source such as a back light device, and thus, consumes less power and can be fabricated thinner and lighter than a light-transmission type liquid crystal display device. A reflection type liquid crystal display device is used mainly in a handy communication terminal.
Hereinbelow is explained a conventional reflection type liquid crystal display device with reference to FIG. 1 which is a plan view of a conventional reflection type liquid crystal display device, FIG. 2 which is a cross-sectional view taken along the line II-II in FIG. 1, and FIGS. 3A to 3H which are cross-sectional views each illustrating a step of a method of fabricating a substrate on which a thin film transistor (TFT) is to be fabricated. FIG. 1 illustrates pixels located at an outer periphery of a display area in which images are to be displayed. Electrode terminals and other parts are formed in areas located at upper and left sides of the illustrated pixels, outside the display area.
First, a structure of a conventional reflection type liquid crystal display device is explained hereinbelow with reference to FIGS. 1 and 2.
The illustrated conventional reflection type liquid crystal display device is comprised of a TFT substrate 5 on which a thin film transistor (TFT) is formed, an opposing substrate 6 facing and spaced away from the TFT substrate 5, and a liquid crystal display layer 4 sandwiched between the TFT substrate 5 and the opposing substrate 6.
The TFT substrate 5 is comprised of gate lines 1, drain lines 2 extending perpendicularly to the gate lines 1, switching devices each comprised of a thin film transistor 3 formed in each of pixel areas defined by the gate lines 1 and the drain lines 2, a reflection electrode 18 which reflects a light entering the pixel areas and applies a voltage to liquid crystal molecules in the liquid crystal layer 4, a first electrically insulating film 16 formed on the TFT substrate 5, and a second electrically insulating film 17 which cooperates with the first electrically insulating film 16 to present a wavy surface to the reflection electrode 18.
The thin film transistor 3 has a gate electrode 11 electrically connected to the gate line 1, a drain electrode 14 electrically connected to the drain line 2, and a source electrode 15 electrically connected to the reflection electrode 18.
As illustrated in FIG. 2, the TFT substrate 5 is comprised further of a first substrate 10 on which the gate electrode 11 is formed, a gate insulating film 12 formed entirely on the first substrate 10, an amorphous silicon layer 13a formed on the gate insulating film 12, and n+ amorphous silicon layers 13b formed on the amorphous silicon layer 13a. 
The drain electrode 14 and the source electrode 15 extend covering both the n+ amorphous silicon layers 13b and the gate insulating film 12 therewith.
The first electrically insulating film 16 is randomly formed in each of pixels in the display area, and is covered with the second electrically insulating film 17 to smooth steps formed by the first electrically insulating film 16. The reflection electrode 18 has a wave surface showing a certain optical reflection characteristic, reflecting a wavy surface of the second electrically insulating film 17.
As illustrated in FIG. 2, the reflection electrode 18 is electrically connected to the source electrode 15 at a contact hole 19.
The opposing substrate 6 is comprised of a second substrate 20, a color filter 21 formed on a first surface of the second substrate 20, a common electrode 22 through which a voltage is applied to liquid crystal molecules in the liquid crystal layer 4, and a polarizing plate 23 formed on a second surface of the second substrate 20.
Liquid crystal molecules in the liquid crystal layer 4 are controlled by a voltage applied across the TFT substrate 5 and the opposing substrate 6.
An incident light 24 passing through the opposing substrate 6 and the liquid crystal layer 4 is reflected at the reflection electrode 18 having the wavy surface, and then, passes again through the liquid crystal layer 4 and the opposing substrate 6, and leaves the liquid crystal display device as an out-going light 25.
In the conventional liquid crystal display device, steep steps formed by the first electrically insulating film 16 randomly formed on the TFT substrate 5 are smoothed by the second electrically insulating film 17 thinner than the first electrically insulating film 16, as mentioned earlier. As a result, the reflection electrode 18 has a sufficiently wavy surface at which the incident light 24 is randomly reflected, ensuring that images can be displayed on a screen with uniform brightness.
Hereinbelow is explained a method of fabricating the TFT substrate 5 in the above-mentioned conventional liquid crystal display device with reference to FIGS. 3A to 3H. The thin film transistor 3 acting as a switching device has a reverse-stagger structure.
First, as illustrated in FIG. 3A, the gate electrode 11 and the gate line 1 are formed on the first substrate 10. Then, the gate insulating film 12 is formed on the first substrate 10, covering the gate electrode 11 therewith. Then, the amorphous silicon layer 13a is formed on the gate insulating film 12 above the gate electrode 11, and subsequently, the n+ amorphous silicon layer 13b is formed on the amorphous silicon layer 13a. 
Then, the drain electrode 14 and the source electrode 15 are formed partially covering the n+ amorphous silicon layer 13b therewith and further partially covering the gate insulating film 12 therewith.
Then, the n+ amorphous silicon layers 13b is etched in its exposed area with the drain and source electrodes 14 and 15 being used as a mask, to thereby fabricate the thin film transistor 3. Then, the thin film transistor 3 is covered with a passivation film (not illustrated).
Then, as illustrated in FIG. 3B, the first electrically insulating films 16 composed of a resin are randomly formed in each of pixel regions. The electrically insulating films 16 are formed to have a thickness equal to or greater than a predetermined thickness in order to provide appropriate optical reflection characteristic to the reflection electrode 18.
Then, as illustrated in FIG. 3C, the first electrically insulating films 16 are heated to turn their sharp corners into rounded corners.
Then, as illustrated in FIG. 3D, the first electrically insulating films 16 are covered with the second electrically insulating film 17. Since the second electrically insulating film 17 is formed in order to smooth steps formed by the first electrically insulating films 16, if it is too thin, the steps formed by the first electrically insulating films 16 remain as they are, and if it is too thick, the second electrically insulating film 17 would have a planar surface. Hence, a thickness of the second electrically insulating film 17 is determined taking the optical reflection characteristic of the reflection electrode 18 into consideration.
Then, the second electrically insulating film 17 is removed in an area outside the display area, and concurrently removed partially above the source electrode 15 to form the contact hole 19 through which the reflection electrode 18 is electrically connected to the source electrode 15.
Then, as illustrated in FIG. 3E, a metal 18b having high reflectivity is deposited all over the first substrate 10.
Then, as illustrated in FIG. 3F, the metal 18b is entirely covered with a resist 26.
Then, as illustrated in FIG. 3G, the resist 26 is exposed to a light and subsequently developed such that a resist pattern 26a covers only an area in which the reflection electrode 18 is to be formed. Then, the metal 18b is etched for removal with the resist pattern 26 being used as a mask.
Thus, as illustrated in FIG. 3H, the reflection electrode 18 composed of the metal 18b is formed covering the second electrically insulating film 17 therewith. Then, the resist pattern 26a is removed.
The resultant reflection electrode 18 is electrically connected to the source electrode 15 in each of pixels. The reflection electrode 18 is removed at a boundary between pixel areas, that is, on both the gate line 1 and the drain line 2, and further in an area (an area located at the left in FIG. 3H) where electrode terminals are to be formed which area is outside the display area, in order that the reflection electrode 18 acts as a pixel electrode to apply a voltage to liquid crystal molecules in the liquid crystal layer 4.
However, the above-mentioned conventional liquid crystal display device and the above-mentioned method of fabricating the same are accompanied with the following problems.
In the step having been explained with reference to FIG. 3D, the second electrically insulating film 17 formed for smoothing the steps formed by the first electrically insulating films 16 is formed also on both the gate line 1 and the drain line 2 between adjacent pixels, in order to make it easy to remove the reflection electrode 18. In the area (which is located at the left in FIG. 3H) where electrode terminals are to be formed, located outside the display area, the second electrically insulating film 17 is removed at the same location as the first electrically insulating film 16, in order to render the area as small as possible and thereby fabricate a liquid crystal display device in a small size. As a result, as illustrated in FIG. 3H, the first and second electrically insulating films 16 and 17 have a steep cross-section at an end thereof.
Herein, it is assumed that the resist 26 is deposited entirely over the first substrate 10 with the first and second electrically insulating film 16 and 17 having a steep cross-section. The resist 26 would have a designed thickness in an area where the second electrically insulating film 17 covers the first electrically insulating film 16 therewith to thereby have a smooth upper surface, that is, in pixels or on the gate line 1 and the drain line 2 between adjacent pixels. In contrast, in the area where electrode terminals are to be formed, located outside the display area, the resist 26 would gather due to the steep cross-section, and resultingly, would have a thickness greater than a designed thickness.
Since the conditions for carrying out exposure of the resist 26 to a light and development of the resist 26 are determined based on a resist existing between pixels which resist is required to be exactly patterned, the resist 26 could not be completely removed at an end of the first and second electrically insulating films 16 and 17 having a great thickness, resulting in resist residue 26b, as illustrated in FIG. 3G.
The resist residue 26b would prevent the metal 18b existing therebelow from being etched, resulting in an non-removed portion 18a of the metal 18b, as illustrated in FIG. 3H.
If the portion 18a of the metal 18b remains not removed in an area where the metal 18b has to be all removed, as mentioned above and as illustrated in FIG. 3H, there would be unintentionally generated a parasitic capacity between the non-removed portion 18a and the gate and drain lines 1 and 2, resulting in remarkable degradation in display quality in the liquid crystal display device.
As an alternative, if the non-removed portion 18a of the metal 18b bridges over adjacent pixels, there would be caused a problem that the resultant reflection electrode 18 falls into short-circuit.